Method and apparatus for automated launch, retrieval, and servicing of a hovering aircraft

ABSTRACT

An aircraft capable of thrust-borne flight can be automatically retrieved, serviced, and launched using equipment suitable for use on a small vessel, or a base with similarly limited space or irregular motion. For retrieval, the aircraft drops a weighted cable, and pulls it at low relative speed into a broad aperture of a base apparatus. Continued translation of the aircraft may pull the cable clear of the apparatus, in which case it can continue in free flight and return for another retrieval attempt. Alternatively, the cable will be dragged along guiding surfaces of the apparatus into and through a slot or similar channel, until its free end is captured. The aircraft, having thus become anchored to the base station, is then pulled down by the cable into a receptacle. Guiding surfaces of the receptacle adjust the position and orientation of a probe on the aircraft, while directing the probe to mate with a docking fixture. Once mated to the fixture, the cable can be released and stored aboard the aircraft; the aircraft can be automatically shut down; and fueling or other servicing can be completed through appropriate connectors in the docking fixture. The aircraft can remain docked as needed, and when desired, be automatically started and tested in preparation for launch. It can then be released into free flight. A full ground-handling cycle can thus accomplished with simple and economical apparatus. It can be used with low risk of damage, and only moderate piloting accuracy.

PRIORITY CLAIM

This patent application is a non-provisional patent application of, claims priority to and the benefit of U.S. Provisional Patent Application Ser. No. 61/152,076, filed Feb. 12, 2009, the entire contents of which are incorporated herein by reference.

BACKGROUND

1. Field of Invention

The present invention addresses launch, retrieval, and servicing of a hovering aircraft, especially in turbulent winds or onto an irregularly-moving surface, such as the deck of a ship in a rough sea. Various embodiments of the present invention are especially suited to unmanned aircraft of small size. These embodiments allow for a fully automated operations cycle, whereby the aircraft can be repeatedly launched, retrieved, serviced, and re-launched, without manual intervention at any point, and while requiring only modest accuracy in piloting.

2. Description of Problem

Hovering aircraft, be they helicopters, thrust-vectoring jets, “tail-sitters”, or other types, usually land by gently descending in free thrust-borne flight onto a landing surface, coming to rest on an undercarriage of wheels, skids, or legs. This elementary technique, can be problematic in certain situations, such as when targeting a small, windswept landing pad on a ship moving in a rough sea. Decades ago, the Beartrap or RAST system was developed to permit retrieval with acceptable safety in such conditions. Retrieval with this system involves securing a line between a helicopter and landing deck, and then winching the helicopter down onto a trolley. The helicopter is fastened to the trolley. After retrieval, the trolley is used to move the helicopter along the deck. The system is effective and widely used, but requires an expensive and substantial plant in the landing area, as well as manual operations coordinated between helicopter and shipboard crew. Furthermore, the helicopter must carry a complete undercarriage in addition to the necessary Beartrap components.

Desirable improvements relative to the Beartrap system include (a) simplification of the apparatus, and (b) automated rather than manual operation. Ideally not only would retrieval but also subsequent refueling and launch would be automated. This would be particularly desirable for an unmanned aircraft, whose operations cycle could then be made fully autonomous. Some experimental work toward this objective has been done for a hovering aircraft, as described in the publication by Mullens et al. titled, “Automated Launch, Recovery, and Refueling for Small Unmanned Aerial Vehicles” (2004); however, success has been limited even with light wind and a stationary base. The present invention by contrast provides for fully automated operation in calm or rough conditions, using apparatus which is simple, portable, and suitable for a small vessel or similarly confined base.

SUMMARY

In one embodiment of the method of the present invention, an aircraft would proceed automatically from free thrust-borne flight to retrieval to launch through the following sequence of actions:

-   -   (a) while approaching a base at low relative speed, the aircraft         drops a weighted cable;     -   (b) the aircraft then flies over a retrieval apparatus, which         brings the cable into an aperture of cable guides, which in one         embodiment forms the shape of a V in the horizontal or         substantially horizontal plane;     -   (c) further translation pulls the cable into and through a slot         at the terminus of the cable guides, which captures the cable;     -   (d) the aircraft is then anchored;     -   (e) if the cable is not captured, the aircraft can climb away         and return for another approach;     -   (f) the aircraft, recognizing capture of the cable by an         increase in tension, winches-in the cable and so draws itself         into a docking receptacle, such as, in one embodiment, a         funnel-like receptacle at the vertex of the cable guides;     -   (g) as the aircraft is drawn into the docking receptacle,         guiding surfaces align and ultimately mate the aircraft with one         or more connectors for docking and servicing;     -   (h) the cable is released from the retrieval apparatus, and         retracted by the aircraft;     -   (i) the aircraft is shut-down, refueled and otherwise serviced         as necessary through one or more suitable connectors;     -   (j) the aircraft completes launch preparations, and develops         sufficient thrust to accelerate away from the retrieval         apparatus when released; and     -   (k) the aircraft is released into thrust-borne free flight.

Since loads can be low during retrieval from hover, the apparatus can be light and portable. Furthermore, easy targeting makes the technique well-suited for both manual control and economical automation.

It should be appreciated that the apparatus of various embodiments of the present invention include an aircraft docking assembly attached to an aircraft, a base retrieval apparatus attached to a stationary or movable base, and the combination of these configured so as to accomplish the methods of the present invention.

Additional features and advantages are described herein, and will be apparent from the following Detailed Description and the Figures.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1A, 1B, 1C and 1D are a series of diagrammatic rear-quarter perspective views of an embodiment of the present invention for a helicopter, illustrating an aircraft docking assembly attached to the helicopter, a base retrieval apparatus or servicing station, and the helicopter sequentially entering, capturing, docked in, and launching from the base retrieval apparatus or servicing station.

FIG. 2 is an enlarged partially fragmentary perspective view of a the base retrieval apparatus or servicing station for capturing, docking, servicing, and launching a helicopter.

FIGS. 3A, 3B, 3C and 3D are a series of diagrammatic rear-quarter perspective views of an embodiment of the present invention for a hovering “tail-sitter” aircraft, illustrating an aircraft docking assembly attached to the aircraft, a base retrieval apparatus or servicing station, and the aircraft sequentially entering, capturing, docked in, and launching from the base retrieval apparatus or servicing station.

FIG. 4 is an enlarged perspective view of a representative docking probe mounted in the tail of a “tail-sitter” aircraft of one embodiment of the present invention.

FIG. 5 is a perspective view of a representative aircraft as in FIG. 4 being pulled into a docking receptacle of the base retrieval apparatus of one embodiment of the present invention.

FIGS. 6A, 6B, 6C and 6D are a series of diagrammatic rear-quarter perspective views of an embodiment of the present invention for a hovering aircraft, illustrating a possible downwind retrieval and launch sequence.

DETAILED DESCRIPTION

Various embodiments of the present invention are generally directed to apparatus and methods for retrieving a flying object or an aircraft from substantially thrust-borne free flight. In one embodiment, the apparatus includes an aircraft docking assembly for a helicopter and a base retrieval apparatus attachable to a stationary or movable base. In another embodiment, the apparatus includes an aircraft docking assembly for an aircraft configured for efficient wing-borne flight and a base retrieval apparatus attachable to a stationary or movable base. It should be appreciated that the present invention is not limited to the embodiments illustrated in the figures and described below, and that in alternative embodiments, the shape, size, configuration and/or arrangement of one or more of the various components described below may vary. It should also be appreciated that the present invention need not include each and every of the components in the embodiments illustrated in the figures and described below.

Referring now to FIGS. 1A, 1B, 1C, 1D and 2, one embodiment of the aircraft docking assembly and base retrieval apparatus for a helicopter are generally illustrated. The base retrieval apparatus includes a base station 5 having a base fuel tank 12 and a base member 33 extending upwardly from the base fuel tank 12. The base station 5 may include an azimuthal pivot 21, as described below. In the illustrated embodiment, the base station 5 also includes support member 34 connected to the base member 33 for supporting a base docking device, fixture or probe receiver 11. A guide, funnel, or funnel like docking receptacle 9 is attached to, and extends upwardly from, the base docking device, fixture or probe receiver 11. The guide, funnel, or funnel like docking receptacle 9 includes guiding surfaces. The guide, funnel, or funnel like docking receptacle 9 has or defines a slot 10 configured to admit a cable 2, as discussed below. The support member 34 includes outwardly extending arms 4. The arms 4 extend outwardly defining an angle. A slot 6 is defined or placed near the vertex of the arms 4. Aerodynamic surfaces or members 22 may be respectively attached to the arms 4.

In one of the illustrated embodiments, the aircraft docking assembly is attached to the helicopter and includes a cable 2, a cable point or fixture such as a cable end fitting 3, a cable length reducer such as a winch 7, and an aircraft docking device or fixture such as a probe 8. The probe includes guiding surfaces and is substantially cylindrically shaped in one embodiment. The probe 8 is attached to the helicopter and extends beyond the skids 26 of the helicopter. At least a portion of the cable 2 is configured to be wound around a drum of the winch 7. In another embodiment, the winch 7 is attached to the base retrieval apparatus as described below.

More specifically, FIGS. 1A, 1B, 1C and 1D show an illustrative embodiment of the present invention, as used with the helicopter 1 of conventional layout. In preparation for retrieval, the helicopter 1 deploys the lightweight cable 2 weighted by the cable end fitting 3, and drags it between the arms 4 of the base station 5. If the helicopter's path falls within a capture envelope—determined by, primarily, the length l_(a), vertex angle ψ_(a), and droop angle ε_(a) of the arms, and the length l_(c) of the cable (and associated height of the servicing apparatus)—then the cable is guided into a cable holder configured to hold the cable 2 (through the slot 6 located at the vertex of the arms 4 as shown in FIG. 2). The helicopter pulls the cable through the slot 6 until further motion is restrained by the cable end fitting 3. The cable end fitting thus anchors the helicopter. In various embodiments, the cable end fitting, cable, or slot may be made compliant to limit shock loading. If the helicopter's path is such that the cable misses the arms entirely, or is pulled over an arm before reaching the slot 6, then the helicopter simply continues in free flight, and can return for another approach.

Once the helicopter is anchored it can increase thrust, and the cable will tend to stay nearly vertical despite disturbances. The helicopter's position can also be regulated by appropriate control, for example of rotor thrust and in-plane moments.

The constraint imposed by the anchored cable can be recognized by the helicopter, and used to trigger the next retrieval step. This involves pulling the helicopter downward toward the base docking device, fixture or probe receiver 11, for example by activating a winch 7 on the helicopter or on the base station. In one embodiment, this causes the probe 8 on the helicopter to enter, and to be guided to the base of, the guide, funnel, or funnel like docking receptacle 9 on the base station. In one embodiment, the funnel incorporates a cable aperture such as a slot 10 to admit the cable, and thus allow for close placement of the cable and probe on the helicopter. The guide, funnel, or funnel like docking receptacle 9 guides the probe 8 to mate or match firmly with the base, docking device, fixture or probe receiver 11, thus completing the retrieval. Mating or matching can be detected by a suitable sensor in the probe or in the base docking device, fixture or probe receiver 11.

Once retrieval is complete, the cable can be released from the capture slot, and optionally retracted into the helicopter. The helicopter's engine can be stopped. Servicing, such as provision of electrical power, refueling from a base supply, and weighing of the aircraft, can be effected through one or more suitable connectors and sensors in the probe 8 and base docking device, fixture or probe receiver 11. The helicopter can remain docked until such time as launch is desired. These connectors can be configured to automatically transfer fluids and/or electricity to the aircraft.

For launch, appropriate self-testing can be completed, and the helicopter then run-up. Release into free flight should be permitted only when thrust is sufficient for positive separation. This condition can be enforced by various ways, such as an appropriately large break-out force in the docking fixture, or a suitable combination of thrust measurement and active triggering of an unlocking device (not shown). The aircraft would extract the cable from the docking fixture through the slot 10 and could then winch it onboard.

Referring now to FIGS. 3A, 3B, 3C, 3D, 4, 5, 6A, 6B, 6C and 6D, one embodiment of an docking assembly and base retrieval apparatus for an aircraft configured for efficient wing-borne flight is generally illustrated. The aircraft includes a fixed wing 17, a propellor 18, a fuselage 31, and an empennage 20. The empennage 20 includes vertical stabilizer 27 and horizontal stabilizers 28. The aircraft docking assembly includes cable 2, cable end fitting 3, aircraft docking device or fixture such as a probe 8, and winch 7. In another embodiment, the winch 7 is attached to the base retrieval apparatus as described below. The probe 8 may include fuel and electrical connectors 13 located at an end portion of the probe 8. A cable guide 32 may be included to guide the cable as it is wound from the drum of the winch 7. In the illustrated embodiment, such a cable guide 32 is formed in the shape of a funnel.

The illustrated base retrieval apparatus for an aircraft configured for efficient wing-borne flight includes base station 5 having a base fuel tank 12 and a base member 33 extending upwardly from the base fuel tank 12. The base station 5 also includes support member 34 connected to the base member 33 for supporting a base docking device, fixture or probe receiver 11. The guide, funnel, or funnel like docking receptacle 9 is replaced by guide or docking receptacle 19, having edges 35 that serve to admit and orient the empennage surfaces 27 and 28 of the aircraft as it is pulled into base docking device, fixture or probe receiver 11, as discussed below. The support member 34 includes arms 4. The arms 4 extend outwardly defining an angle. A slot 6 is defined or placed near the vertex of the arms 4. Aerodynamic surfaces or members 22 may be respectively attached to a portion of the arms 4. In one embodiment, the base station 5 may include an azimuthal pivot 21, as described below.

FIG. 3 shows the aircraft 16 having a configuration suited to efficient wing-borne flight. A propeller 18 is installed at its nose, with the propeller's spin axis aligned with the fuselage 31. The winch 7 and probe 8, which are comparable to those in FIGS. 1A, 1B, 1C and 1D and FIG. 2, are mounted at the rear of the fuselage 31, as shown in more detail by FIG. 4 and described above. To prepare for retrieval, the aircraft pitches up from wing-borne flight, with its thrust line near horizontal, into thrust-borne flight, with its thrust line near vertical. Rotor thrust is adjusted to balance aircraft weight. The cable 2 is then deployed, and retrieval proceeds much as was described for the helicopter of FIGS. 1A, 1B, 1C and 1D and FIG. 2. In this case, however, the guide or docking receptacle 9 of FIGS. 1A, 1B, 1C and 1D and FIG. 2 is replaced by a guide or docking receptacle 19 in the form of a set of petals whose edges 35 serve to admit and orient the empennage surfaces 27 and 28 of the aircraft as its probe 8 is pulled into the base docking device, fixture or probe receiver 11, as illustrated by FIG. 5. Thus, the combination of an appropriately long cable 2, appropriately open arms 4, and appropriately shaped petals, permits successful retrieval across a wide range of aircraft approach paths and orientations. After retrieval, the aircraft can be serviced and re-launched much as was described for the helicopter of FIGS. 1A, 1B, 1C and 1D and FIG. 2.

For automated retrieval, the aircraft and base retrieval apparatus each can be equipped with a suitable device for measuring relative position and velocity in three dimensions, such as satellite-navigation equipment having antennas on the aircraft 14 and on a reference point such as point 15 near the base docking device, fixture or probe receiver 11. In an embodiment, each of the aircraft and base retrieval apparatus can also have equipment for measurement of orientation, such as magnetic or inertial sensors, as well as appropriate mechanisms for computation, power supply, and communication.

Communication between the aircraft and base retrieval apparatus can also be used, for example, to trigger the base retrieval apparatus to release the cable in the event of an anomaly, such as an excessive mismatch in position or orientation as the aircraft is pulled toward the base docking device, fixture or probe receiver 11. In that case, the aircraft would fly clear of the base station and could return for another approach.

In many cases, the preferred approach direction will vary with wind velocity. This can be accommodated by providing a base retrieval apparatus including a base station mounted on the azimuthal pivot 21 (as shown in FIG. 2). The base support member 34 could then be oriented or rotated by a suitable actuator on the pivot, or by the weathervane action of the suitably placed aerodynamic surfaces or members 22.

In light to moderate wind, the preferred approach direction would typically be upwind. However, if the wind speed V_(W) were to exceed the maximum airspeed V_(A,max) at which an aircraft such as that shown in FIGS. 3A, 3B, 3C and 3D could sustain level thrust-borne flight, then an upwind approach would be possible only while descending. For an approach in level flight, the procedure illustrated in FIGS. 6A, 6B, 6C and 6D would be used instead. In this case, the aircraft would fly into the wind at a designated airspeed V_(A), while drifting downwind toward the base station at speed (V_(W)-V_(A)). Capture of the cable would proceed as described for FIGS. 1A, 1B, 1C and 1D and FIGS. 3A, 3B, 3C and 3D; however, once anchored, the aircraft would not be able to hover vertically above the base docking receptacle. Instead, the aircraft could hover, and so maintain line tension, only in a downwind kite-like position as shown in FIG. 6B.

To accommodate this situation, the base docking device, fixture or probe receiver and the guide or docking receptacle may be mounted on a gimbal 23 so that the axis of the funnel can align with the cable, as shown in FIG. 6B. The gimbal could be set as desired after the aircraft mated to the base docking device, fixture or probe receiver, typically to thrust-vertical orientation. The torque necessary thus to orient the gimbal can be provided by the aircraft itself, or by an actuator on the base station. Once set at the desired orientation, the gimbal can be locked in place by an appropriate mechanism.

For launch in a strong wind, a downwind gimbal tilt may likewise be necessary for the aircraft to accelerate out of the base docking device, fixture or probe receiver upon release. In preparation for such a downwind launch, the gimbal can be unlocked and tilted as appropriate. The aircraft can then pull itself out of the base docking device, fixture or probe receiver as shown in FIG. 6C. Once clear, the aircraft could reorient if desired to reduce the downwind drift rate, as shown in FIG. 6D. An anemometer 24 on the base station can be used to select the appropriate orientation for launch.

It should be understood that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present subject matter and without diminishing its intended advantages. It is therefore intended that such changes and modifications be covered by the appended claims. 

1. A method for retrieving a flying object from free flight onto a base retrieval apparatus, said method comprising (a) hanging a cable from the flying object; (b) translating the cable, by motion of the flying object relative to the base retrieval apparatus, into a horizontal or substantially horizontal aperture formed by one or more linear or curvilinear guiding surfaces of the base retrieval apparatus and then into contact with a cable holder configured to hold a point or fixture on the cable to the base retrieval apparatus; (c) holding the point or fixture on the cable by the cable holder, thereby constraining motion of the flying object relative to the base retrieval apparatus; (d) while maintaining tension in the cable substantially with thrust on the flying object, pulling the flying object toward the base retrieval apparatus using the cable, during the course of which at least one guiding surface of an aircraft docking device attached to the flying object and at least one guiding surface of the base retrieval apparatus guide the aircraft docking device into a base docking device of the base retrieval apparatus; and (e) holding the flying object firmly in position relative to the base retrieval apparatus to complete the retrieval.
 2. The method of claim 1, which includes releasing the cable from the cable holder and storing the cable on the flying object prior to re-launching the flying object.
 3. The method of claim 1, which includes causing the flying object to release itself or pull itself free from the base retrieval apparatus to re-launch into free flight.
 4. The method of claim 1, which includes causing the flying object to continue in free flight if the point or fixture on the cable fails to be captured by the cable holder.
 5. The method of claim 1, which includes rotating the one or more linear or curvilinear guiding surfaces of the base retrieval apparatus to accommodate variations in approach direction of the flying object.
 6. The method of claim 1, which includes tilting the base docking device of the base retrieval apparatus to accommodate variations in the orientation of the cable between cable holder and the flying object to facilitate alignment and matching of the aircraft docking device and base docking device.
 7. The method of claim 1, which includes, during or after matching of the aircraft docking device and base docking device, making on one or more connections to transfer fluid or electricity between the flying object and the base retrieval apparatus.
 8. The method of claim 7, which includes making the connections and transferring fluid or electricity.
 9. The method of claim 1, which includes causing the cable, the point or fixture on the cable, or the cable holder to hold the point or fixture on the cable to comply to moderate loads in the cable.
 10. The method of claim 1, which includes measuring a three-dimensional position of the flying object relative to the base retrieval apparatus.
 11. The method of claim 1, wherein the base retrieval apparatus is attached to a moving vehicle.
 12. An apparatus for retrieving an aircraft from substantially thrust-borne free flight, said apparatus comprising: a aircraft docking assembly including: a cable configured to be hung below the aircraft, the cable having a point or fixture, and an aircraft docking device having at least one guiding surface; a base retrieval apparatus including: a cable holder configured to hold the point or fixture on the cable to the base retrieval apparatus, one or more linear or curvilinear cable-guiding surfaces forming a horizontal or substantially horizontal aperture configured to admit the cable and to guide the cable into the cable holder, a docking receptacle, and a base docking device; and a cable length reducer attached to the aircraft or the base retrieval apparatus and configured to reduce the length of the cable between the aircraft and the cable holder, while maintaining tension on the cable, thus drawing into contact the guiding surfaces of the aircraft docking device and the docking receptacle, and such that as the distance between the cable holder and the aircraft is reduced, the guiding surfaces of the aircraft docking device and the docking receptacle direct the aircraft docking device into the base docking device, leading to the aircraft being held firmly in position relative to the base retrieval apparatus when the aircraft docking device and the base docking device are mated.
 13. The apparatus of claim 12, wherein the cable holder is configured to release the cable so that the aircraft can pull the cable clear of the base retrieval apparatus and proceed in free flight.
 14. The apparatus of claim 12, wherein the aircraft is configured to be released by or to pull itself free from the base docking device to launch into free flight.
 15. The apparatus of claim 14, wherein the aircraft is configured to be released by or to pull itself free from the base docking device only when thrust is sufficient to lift the aircraft safely clear of the base retrieval apparatus.
 16. The apparatus of claim 12, wherein the one or more linear or curvilinear guiding surfaces of the base retrieval apparatus which form the aperture are rotatable about a vertical or principally vertical axis to accommodate variations in approach direction of the aircraft.
 17. The apparatus of claim 12, wherein the base docking device and the docking receptacle are tiltable to accommodate variations in the orientation of the cable between the cable holder and the aircraft, or to set the appropriate aircraft orientation for launch.
 18. The apparatus of claim 12, which includes one or more connectors configured to be automatically connected and disconnected to the aircraft to transfer fluid or electricity between the aircraft and the base retrieval apparatus.
 19. The apparatus of claim 12, which includes compliance in the cable, the point or fixture on the cable, or the cable holder configured to hold the point or fixture on the cable, under moderate loads in the cable during retrieval.
 20. The apparatus of claim 12, which includes a measuring device configured to measure a three-dimensional position of the aircraft relative to the base retrieval apparatus.
 21. The apparatus of claim 12, which includes a measuring device configured to measure an orientation of the aircraft relative to the base retrieval apparatus.
 22. The apparatus of claim 12, wherein the aircraft docking device is attached to a fuselage of the aircraft.
 23. The apparatus of claim 12, wherein the cable holder includes a one-way gate configured to permit entry of the cable into, but to prevent inadvertent release of the cable from the cable holder.
 24. An aircraft base retrieval apparatus for retrieving an aircraft from free flight, said apparatus comprising: a cable holder configured to hold a point or fixture on a cable attached to an aircraft; one or more linear or curvilinear cable-guiding surfaces forming a horizontal or substantially horizontal aperture configured to admit the cable and to guide the cable into the cable holder; a docking receptacle; and a base docking device configured to mate with an aircraft docking device attached to the aircraft.
 25. The aircraft base retrieval apparatus of claim 24, wherein the cable holder is configured to release the cable.
 26. The aircraft base retrieval apparatus of claim 24, wherein the one, or more linear or curvilinear guiding surfaces which form the aperture are rotatable about a vertical or principally vertical axis to accommodate variations in approach direction of the aircraft.
 27. The aircraft base retrieval apparatus of claim 24, wherein the base docking device and the guiding surfaces are tiltable to accommodate variations in the orientation of the cable between the cable holder and the aircraft, or to set the appropriate aircraft orientation for launch.
 28. The aircraft base retrieval apparatus of claim 24, which includes one or more connectors, which can be connected and disconnected to the aircraft to transfer fluid or electricity to the aircraft.
 29. The aircraft base retrieval apparatus of claim 24, which includes a measuring device configured to measure a three-dimensional position of the aircraft relative to the aircraft base retrieval apparatus.
 30. The aircraft base retrieval apparatus of claim 24, which includes a measuring device configured to measure an orientation of the aircraft relative to the aircraft retrieval base retrieval apparatus.
 31. The aircraft base retrieval apparatus of claim 24, wherein the cable holder includes a one-way gate configured to permit entry of the cable into, but to prevent inadvertent release of the cable from the cable holder. 